The invention relates generally to an electrophotographic printing machine and, more particularly, to a development system which includes a dual auger assembly for mixing the developer.
Generally, an electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitive the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is formed on the photoconductive member, the image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attached to the latent image from the carrier granules to form a powder image on the photoconductive member which is subsequently transferred to a copy sheet. Finally, the copy sheet is heated to permanently affix the powder image thereto in image configuration.
As the toner particles are depleted from the developer material, it is necessary to dispense additional toner particles into the developer mixture. Then newly added toner is typically mixed in some manner with the denuded carrier particles and unused developer material. Various prior art devices have been devised to accomplish the mixing function. A preferred system for accomplishing the crossmixing function is the use of a dual auger system to transport the toner in two directions and achieve a toner interchange between augers. Dual auger systems are disclosed, for example, in the following prior art documents. U.S. Pat. No. 4,274,362 to Beck et al discloses magnetic brush mixing augers made of twisted strips of aluminum sheet metal with smooth axial edges. In a developing unit, the auger members are located in the sump portion of a developing pan where they circulate, distribute and intermix dry toner. A dispensing system evenly distributes regular amounts of toner while the copier is operable.
U.S. Pat. No. 4,056,076 to Smith, assigned to Xerox Corporation, discloses a crossmixing system for mixing and charging multicomponent developer in a circulating development system of an electrostatographic processor. A pair of parallel passive crossmixers are used as mixing devices and a single active crossmixer is used as a blending (triboelectric charging) device.
U.S. Pat. No. 4,146,323 to Forward et al, assigned to Xerox Corporation, discloses an auger for a development system comprised of an elongated twisted strip of sheet metal with helically contoured edges. As toner is dispensed, fresh toner is added to the developer from a toner dispenser directly above a crossmixer to keep the toner concentration at a high level.
U.S. Pat. No. 4,478,512 to Zoltner, assinged to Xerox Corporation, discloses a developer system in which a pair of augers mix newly dispensed toner with denuded carrier particles and returns the mixture into a developer sump.
U.S. Pat. No. 3,999,514 to Abbott et al describes a supply and return auger system in which the augers are rotated at different flute and pitch related speeds which ensure equal flow through the auger.
U.S. Pat. No. 3,664,299 to Shaler et al discloses still another dual auger mixing system.
These prior art patents described above are representative of the dual auger crossmixing type of system. The common characteristic of these systems is that the axis of each auger pair lie essentially in the same horizontal plane with developer exchange between each auger taking place at end locations. A problem with this inter-auger trasnfer is that the developer is exchanged by a sideways pushing application which requires that the augers be physically close to each other. For some systems, this proximity requirement may present a space or geometry problem. A second difficulty with this "push" inter-auger transfer is the tendency for the developer to "bunch up" at the transfer end, sometimes resulting in toner spilling over into other areas of the developer housing unless specific seals are placed at strategic locations.
The present invention is, therefore, directed to a dual auger crossmixing system which accomplishes inter-auger developer transfer by a construction which utilizes the forces of gravity to accomplish the transfer at each auger end. By allowing the force of gravity to act on the developer as it exits an auger end, and in combination with appropriate side baffles to direct developer travel, the auger can be spaced physically further apart than is possible with the prior art systems. Because of the "drop distance" between each auger end, no developer buildup is experienced at either end. More particularly, the invention is directed toward a development system including a developer roll adapted for depositing developer material on an imaging surface having an electrostatic latent image thereon, a dual auger system for mixing said developer material and transferring mixed developer material to said developer roll, said dual auger system comprising a first auger aligned in a horizontal plane and adapted to mix and supply developer material to said developer roll, a second auger, adjacent to said first auger and positioned along a nonhorizontal plane so that adjacent ends of said first and second auger are separated by a vertical distance, said augers having a plurality of developer material transport communication apertures at said auger ends whereby developer material is transported from one auger to the other by gravitational forces.